This invention relates to a variable valve timing system for an internal combustion engine. More particularly, this invention relates to a torque pulse actuated, hydraulic variable valve timing system of the foregoing type with locking capabilities to lock the components of the system in a fixed condition of operation during intervals of low hydraulic pressure, such as during engine start-up.
U.S. Pat. No. 5,107,804 (Becker et al.), which is assigned to the assignee of this application, the disclosure of which is incorporated by reference herein, describes a vane-type, camshaft torque pulse actuated hydraulic camshaft or valve timing system for an internal combustion engine in which the hydraulic fluid that operates the camshaft phase shifting system is engine oil. Such a system has many operating advantages over other known types of valve or camshaft timing systems, for example, in the timeliness of response to changes in engine operating conditions. However, such systems tend to be noisy or otherwise unstable during periods of low engine oil pressure, which can often occur during engine start-up and can occasionally occur during other types of operating conditions. During these times, it is important to be able to lock the otherwise relatively movable components of the system into fixed positions relative to one another, and it is to the provision of an improved solution of the system locking requirements of such a variable valve timing system that the present invention is directed.
U.S. Pat. No. 2,861,557 (Stolte) also describes a hydraulic variable camshaft timing system, albeit a system that is operated solely by engine oil pressure. This reference teaches that it is desirable to lock the otherwise variable components of the system in fixed positions relative to one another during low speed operation conditions, but only teaches a system in which a single set of fixed positions can be achieved.
The invention of the aforesaid U.S. Pat. No. 6,250,265 is a hydraulic variable camshaft timing system with a locking plate that is spring biased against engine oil pressure from a position out of locking engagement with a variable position camshaft phaser housing when engine oil pressure is high, and into a locking position with the phaser housing when engine oil pressure is low. The locking plate of this invention has a multitude of teeth in an annular array, and the teeth of the locking plate engage the teeth of the phaser housing when the locking plate is in locking engagement with the phaser housing. Because the locking plate and the phaser housing each have a multitude of engageable teeth, the locking plate can lockingly engage the phaser housing at a multitude of relative circumferential positions between the locking plate and the phaser housing. The ability of the invention of U.S. Pat. No. 6,250,265 to lock the phaser housing in any of a multitude of relative positions beneficially reduces phaser oil consumption and phaser oscillation or dither, and it reduces the control system operating requirements from fulltime, in a system where there is no locking capability, to part-time.
It is known in the art that any camshaft phaser locking arrangement must be torsionally rigid, but axially flexible, to reliably engage and disengage when required. It must also be able to operate with minimal backlash between the locking plate and the phaser housing.
A variable camshaft timing system according to the present invention is, like the system of the aforesaid U.S. Pat. No. 6,250,265, a camshaft torque pulse actuated, engine oil powered hydraulic system that is used to change the position of a lobed vane-type rotor within lobe receiving recesses of a surrounding housing. Further, the variable camshaft timing system of the present invention, like that of the aforesaid U.S. Pat. No. 6,250,265, has a locking plate that is spring biased against the effects of engine oil pressure, to prevent relative motion between the rotor and the housing, except when the engine oil pressure exceeds a predetermined value, and locking according to the present invention, like that of the aforesaid U.S. Pat. No. 6,250,265, can occur at one or another of a multitude of positions of the rotor and the housing relative to one another. It is also contemplated that the invention of the present application can be adapted to an hybrid variable camshaft timing system operated both on engine oil pressure and oil pressure resulting from camshaft torque pulses, such as that of U.S. Pat. No. 5,657,725 (Butterfield et al.), which is also assigned to the assignee of this application, the disclosure of which is also incorporated by reference herein, and to an engine oil pressure activated system such as that of the aforesaid U.S. Pat. No. 2,861,557.
Unlike the invention of the aforesaid U.S. Pat. No. 6,250,265 however, the locking plate of the present invention is physically connected to the lobed rotor of the variable camshaft timing phaser, and this connection is by way of a plurality of straps, preferably three straps, which are arranged in an annular array, opposed ends of each of which are secured, for example, by riveting or bolting, to the locking plate and the lobed rotor, respectively. The straps are oriented so that, in their non-locking positions of the locking plate and the rotor of the phaser, they serve to bias the locking plate toward the rotor against the effects of engine oil pressure acting on the locking plate. Thus, when the engine oil pressure falls below an acceptable minimum, an on/off solenoid, which controls the oil pressure, turns off, and the straps bias the locking plate into locking position with the rotor of the phaser.
The strap connection between the locking plate and the rotor of the phaser is axially flexible, in that it permits rapid engagement and disengagement between such elements and it is also circumferentially rigid, to thereby prevent relative circumferential motion therebetween. Further, the strap connection between the locking plate and the phaser rotor makes it possible to eliminate or to substantially eliminate backlash between the locking plate and the rotor, compared to that achieved by the invention of U.S. Pat. No. 6,250,265, by providing wedge-shaped interengaging teeth on the locking plate and the rotor. This form of engagement is a very low backlash form of engagement in comparison to engagement by interengaging spline teeth.
Accordingly, it is an object of the present invention to provide an improved vane-type, torque pulse actuated, hydraulic variable valve timing, or variable camshaft timing system for an internal combustion engine. More particularly, it is an object of the present invention to provide a variable valve timing or variable camshaft timing system of the foregoing character, with an improved arrangement for locking a position of a lobed rotor relative to the position of the housing, in which the rotor is normally free to oscillate whenever engine operating conditions make it desirable to prevent relative motion between the rotor and the housing. Even more particularly, it is an object of the present invention to provide a variable valve timing or a variable camshaft timing system of the foregoing character that eliminates or virtually eliminates backlash between a locking plate of the system and the rotor thereof.
For a further understanding of the present invention and the objects thereof, attention is directed to the drawings and the following brief description thereof, to the detailed description of the preferred embodiment and to the appended claims.